According to the RS232 standard, dropping DTR from active to inactive for at least two seconds tells the modem to disconnect (end) a call or data connection.
When a serial connection is made between two computers using a null modem adapter, the DTR and the Data Carrier Detect (DCD) lines are typically paired.
On many operating systems, including Windows, the DTR line is held low while the serial port is unused and not being controlled by any applications.
On some printers with serial interfaces, the DTR line is used for hardware flow control, similar to the use of RTS and CTS for modems.
However, because during printing, the bulk of the data is from the computer to the printer, the importance of flow control in the opposite direction is minimal.
The mouse driver holds the DTR and RTS lines high at all times so that the device has a source of power.